Top Of Band Research Articles

Realization of robust Ohmic contact for semiconducting black arsenic by coupling with graphene

Abstract Ohmic contacts are fundamental components in semiconductor technology, facilitating efficient electrical connection and excellent device performance. We employ the first principles calculations to show that semimetallic graphene is a natural Ohmic contact partner of monolayer semiconducting black arsenic (BAs), for which the valence band top is below the Fermi energy on the order of 102 meV. Thanks to the giant Stark effect induced from the van der Waals electron transfer from BAs to graphene, without destroying their respective band features. Remarkably, we show that this intrinsic Ohmic contact remains robust across a wide range of interlayer distance (adjustable by strain) or vertical electric field, whereas the weak spin splitting of 1 meV order induced by symmetry breaking plays little role in Ohmic contact. These findings reveal the potential applications of graphene-BAs in ultralow dissipation transistors.

Strain‐Engineered Level Alignment in the MoTe2/WSe2 Heterobilayer

When stacked into heterostructures, layered transition metal dichalcogenides (TMDCs) exhibit peculiar electronic and optical properties that may differ substantially from those of their constituents and that largely depend on the level alignment. For example, the MoTe2/WSe2 heterobilayer exhibits a type I band lineup, which can be exploited in emitting devices but limits charge separation. In this first‐principle study based on density functional theory and many‐body perturbation theory, strain is proposed as an effective means to make MoTe2/WSe2 a type II heterostructure. By exploring several configurations where biaxial strain is applied to either or both monolayers, the top of the valence band and the bottom of the conduction band are found at different points in k‐space leading to an indirect‐to‐direct bandgap transition when the lattice constant of WSe2 is expanded by 3.2% or more. In terms of optical properties, all considered systems exhibit a first dark excitation, consistently shifting in energy with the direct electronic gap, while the absorption onset does not vary significantly with strain. Our findings suggest strain as a powerful tool for fine tuning the electronic and optical properties of TMDC heterostructures while preserving their fundamental characteristics, thus opening new avenues for designing optoelectronic applications.

Electronic properties of corundum-like Ir2O3 and Ir2O3-Ga2O3 alloys

In the hexagonal, corundum-like structure, α-Ga2O3 has a bandgap of ∼ 5.1 eV, which, combined with its relatively small electron effective mass, high Baliga's figure of merit, and high breakdown field, makes it a promising candidate for power electronics. Ga2O3 is easy to dope n-type, but impossible to dope p-type, impeding the realization of some electronic device designs. Developing a lattice-matched p-type material that forms a high-quality heterojunction with n-type Ga2O3 would open new opportunities in electronics and perhaps optoelectronic devices. In this work, we studied Ir2O3 as a candidate for that purpose. Using hybrid density functional theory calculations we predict the electronic band structure of α-Ir2O3 and compare that to α-Ga2O3, and study the stability and electronic properties of α-(IrxGa1−x)2O3 alloys. We discuss the band offset between the two materials and compare it with recently available experimental data. We find that the Ir d bands that compose the top of the valence band in α-Ir2O3 are much higher in energy than O p bands in α-Ga2O3, possibly enabling effective p-type doping. Our results provide an insight into using the Ir2O3 or Ir2O3-Ga2O3 alloys as p-type material lattice-matched to α-Ga2O3 for the realization of p–n heterojunctions.

Fe-N Co-Doped BiVO4 Photoanode with Record Photocurrent for Water Oxidation.

Bismuth vanadate (BVO) ranks among the most promising photoanodes for photoelectrochemical (PEC) water splitting. Nonetheless, slow charge separation and transport, besides the sluggish water oxidation kinetics, are key barriers to its photoefficiency. Here, we present a co-doping strategy that significantly improves the charge separation performance of BVO photoanodes. We found that, under standard one sun illumination, the Fe-N co-doped BVO photoanode (Fe-N-BVO) by N-coordinated Fe precursor reaches a record photocurrent density of 7.01 mA cm-2 at 1.23 V vs RHE after modified a surface co-catalyst (FeNiOOH), and exhibits an outstanding stability. By contrast, much lower photocurrent density is obtained for the N-doped, Fe-doped and Fe/N-doped BVO photoanode with separated N and Fe precursors. The detailed experimental characterizations show that the high activity of the Fe-N co-doped BVO photoanode is attributed to the enhanced photo-induced bulk charge separation, as well as the accelerated surface water oxidation kinetics. XPS, EXAFS and DFT calculations clearly show that, instead of formation of deep trapping state in the individually doped BVO, the co-doping of Fe-N into BVO generates Fe-based electronic states just below the bottom of conduction band and N-derived states just above the top of valence band. Such modulations in electronic structure enable the efficient trap of the electrons and holes to enhance the separation of photo-induced carriers, but hinder the charge recombination originated from the deep trapping sites.

Fe−N Co‐Doped BiVO4 Photoanode with Record Photocurrent for Water Oxidation

AbstractBismuth vanadate (BVO) ranks among the most promising photoanodes for photoelectrochemical (PEC) water splitting. Nonetheless, slow charge separation and transport, besides the sluggish water oxidation kinetics, are key barriers to its photoefficiency. Here, we present a co‐doping strategy that significantly improves the charge separation performance of BVO photoanodes. We found that, under standard one sun illumination, the Fe−N co‐doped BVO photoanode (Fe−N−BVO) by N‐coordinated Fe precursor reaches a record photocurrent density of 7.01 mA cm−2 at 1.23 V vs RHE after modified a surface co‐catalyst (FeNiOOH), and exhibits an outstanding stability. By contrast, much lower photocurrent density is obtained for the N‐doped, Fe‐doped and Fe/N‐doped BVO photoanode with separated N and Fe precursors. The detailed experimental characterizations show that the high activity of the Fe−N co‐doped BVO photoanode is attributed to the enhanced photo‐induced bulk charge separation, as well as the accelerated surface water oxidation kinetics. XPS, EXAFS and DFT calculations clearly show that, instead of formation of deep trapping state in the individually doped BVO, the co‐doping of Fe−N into BVO generates Fe‐based electronic states just below the bottom of conduction band and N‐derived states just above the top of valence band. Such modulations in electronic structure enable the efficient trap of the electrons and holes to enhance the separation of photo‐induced carriers, but hinder the charge recombination originated from the deep trapping sites.

Depth profiles of hole traps in the tail region of Al ion implantation into p-type 4H-SiC

Abstract Hole traps generated in the tail region of Al ion implantation in p-type 4H-SiC were characterized by deep level transient spectroscopy measurements. Hole traps energetically located at Ev + 0.51 eV, Ev + 0.72 eV, Ev + 0.77 eV, and Ev + 1.40 eV (Ev : energy of the valence band top) were detected. The hole trap densities were roughly 7–40 times smaller than the implanted Al atom density in the tail region, and the densities of the observed traps exponentially decreased with the decay lengths of 84–150 nm.

Three Metal Complex‐Decorated Halobismuthates: Syntheses, Structures, Semiconducting Behaviors and Theoretical Investigations

AbstractEmploying metal complex cations as the structural modifiers, three new numbers of hybrid halobismuthates family, namely [NH4][Co(phen)3]BiI6 (phen=1,10‐phenanthroline; 1), [Zn(phen)2Br][Bi(phen)Br4] ⋅ H2O (2) and [Cu(2,2′‐bipy)2Br][Cu(2,2′‐bipy)2Bi2Br9] ⋅ H2O (2,2′‐bipy=2,2′‐bipyridine; 3) have been solvothermally constructed and structurally characterized. X‐ray crystallography analyses show that all compounds characteristic of abundant weak interactions feature the discrete anionic motifs. The obtained hybrid materials possess the semiconducting nature, with the visible light responding optical band gaps of 2.13–2.71 eV. Under the alternating light irradiation, they exhibit excellent photoelectric switching properties, whose photocurrent densities (0.85, 0.24, and 0.43 μA/cm2 for compounds 1–3, respectively) compare well with some high‐performance halobismuthate competitors. Further theoretical calculations of compound 1 reveal that the photosensitive [Co(phen)3]2+ cations contribute greatly to the bottom of conduction bands (CBs) and the top of valence bands (VBs), which may be the important reason for its excellent photoelectric behavior. The analyses of Hirshfeld surface, thermal stability, and X‐ray photoelectron spectroscopy were also discussed detailedly in this paper.

Visualization of spin-polarized surface resonances in Pb-based ternary topological insulators

The formation of the topological surface state originates from bulk band inversion at the top of the valence band and the bottom of the conduction band. The transition between normal and topological insulators is known as a topological phase transition. Here we show spin-polarized electronic states of Pb-based ternary topological insulators Pb(Bi1-xSbx)2Te4 (x = 0.55, 0.70, 0.79) investigated by spin- and angle-resolved photoemission spectroscopy and first-principles calculations. We visualize a pair of spin-polarized surface resonances dispersing along the upper edge of projected bulk bands in occupied states. Interestingly, a branch of the spin-polarized surface resonances continuously connects to the topological surface state. The coexistence of the topological surface state and the spin-polarized surface resonances can be explained by considering the topological phase transition.

Interplay between metavalent bonds and dopant orbitals enables the design of SnTe thermoelectrics

Engineering the electronic band structures upon doping is crucial to improve the thermoelectric performance of materials. Understanding how dopants influence the electronic states near the Fermi level is thus a prerequisite to precisely tune band structures. Here, we demonstrate that the Sn-s states in SnTe contribute to the density of states at the top of the valence band. This is a consequence of the half-filled p-p σ-bond (metavalent bonding) and its resulting symmetry of the orbital phases at the valence band maximum (L point of the Brillouin zone). This insight provides a recipe for identifying superior dopants. The overlap between the dopant s- and the Te p-state is maximized, if the spatial overlap of both orbitals is maximized and their energetic difference is minimized. This simple design rule has enabled us to screen out Al as a very efficient dopant to enhance the local density of states for SnTe. In conjunction with doping Sb to tune the carrier concentration and alloying with AgBiTe2 to promote band convergence, as well as introducing dislocations to impede phonon propagation, a record-high average ZT of 1.15 between 300 and 873 K and a large ZT of 0.36 at 300 K is achieved in Sn0.8Al0.08Sb0.15Te-4%AgBiTe2.

Advanced Computational Insights Into the Optical, Electronic, and Thermoelectric Characteristics of Novel Rare‐Earth Ternary Chalcogenides

ABSTRACTRare‐earth ternary materials are distinguished by their tunable optoelectronic characteristics and high thermal stability. First principles computations examine the intricate interaction of novel rare‐earth‐based ternary chalcogenide's electronic, optical, and thermoelectric properties. The spin‐down channel of PrHSe exhibits a substantial energy gap resulting in half‐metallic behavior. The f orbitals of Pr and Er play an important role in forming bonds with Se and H atoms, contributing significantly to the valence band. The preponderance of Pr‐f and Er‐f orbitals near the top of the valence band indicate that electrons in these orbitals are the most energetic and participate in bonding interactions within these materials. The ErHSe has a greater absorption rate than PrHSe, and both materials behave isotropically in the xx and zz directions. The highest peaks of the reflection coefficient (50%–70%) in the 1.0–13.8 eV range suggested a significant level of UV reflectivity. The PrHSe has a higher intrinsic carrier concentration for conduction than ErHSe. At lower temperatures, carrier concentrations increase due to thermal activation processes, improving the Seebeck coefficient in these materials.

Understanding the atomistic behavior of small molecules (O2 and N2) on monometallic M13 nanoparticles

Both experimental data and density functional theory (DFT) calculations clearly indicate that the reactivity of metal clusters for NO is determined by the energy and orbital type (4d or 5s) of the valence band top. Here, we explore this correlation for the reactivity of M13 nanoclusters, being M = Ag, Au, Co, Cu, Fe, Ir, Ni, Os, Pd, Pt, Rh and Ru and adsorbed diatomic molecules, O2 or N2. The possible adsorption configurations, interatomic distances, and adsorption energies for O2 and N2 on M13 clusters have been analyzed in detail.

Amino‐Arsine and Amino‐Phosphine Based Synthesis of InAs@InP@ZnSe core@shell@shell Quantum Dots

AbstractA colloidal synthesis protocol is demonstrated for InAs@InP core@shell quantum dots (QDs) with a tunable InP shell thickness (ranging from 3 to 8 monolayers), utilizing tris(diethylamino)‐arsine and ‐phosphine. Structural analysis reveals that the InP shell preferentially grows onto the tetrahedral InAs cores along the <‐1‐1‐1> directions, forming tetrapodal‐shaped InAs@InP QDs. Growth of the InP shell causes a red shift in the absorption spectrum of the QDs. This is explained by considering that electrons are delocalized throughout the whole core@shell QDs, while holes preferentially leak along the <‐1‐1‐1> directions, as indicated by the density functional theory calculations. This means such heterostructures cannot be described as type‐I or quasi type‐II, contrary to earlier assumptions. The overlap of carrier wavefunctions throughout the entire InAs@InP QD structure results in no significant reduction of the Auger recombination rate, which remains as fast as in InAs QDs. However, the InP shell enhances photoluminescence (PL) efficiency (up to ≈13%) by passivating surface trap states of the InAs QDs (mainly located close to the top of the valence band). The overgrowth of a ZnSe shell endows the QDs with a high PL efficiency (≈55%) and good stability upon air exposure (≈80% PL intensity retention after 14 days).

Does basic physician trainee selection predict successful progression in the Royal Australasian College of Physicians adult medicine training programme? A retrospective cohort study.

The optimal selection process for basic physician trainees (BPTs) is unclear. To assess an adult BPT selection process, including a standardised recruitment interview assessing clinical decision-making and situational judgement. A retrospective multi-year cohort study of applicants who were selected for interview during the annual recruitment for 2011-2015 clinical years in an adult basic physician training network in NSW. The predictive capacity of the final recruitment assessment score was compared with success in the Royal Australasian College of Physicians (RACP) adult medicine clinical examination within 2 years. Four hundred thirty-six applicants were selected for interview (84-91 in each year). Summary mean post-interview scores ranged from 1.25 to 10, with the median ranging between 7 and 8. Median scores were higher in those who passed the examination than those who did not (7.75 vs 6.43, P < 0.0001). Those who did not sit for the examination in the relevant year or never sat for the examination had similar median scores (7 vs 6.75, P = 0.11). Those not deemed eligible for recruitment were less likely to pass the clinical examination within 2 years than those on the eligibility list (odds ratio (OR) = 0.38, 95% confidence interval (CI) = 0.25-0.57). Applicants with mean scores lower than 8 were less likely to pass than those in the top band (8.1-10); scores between 6.1 and 8 OR 0.42 (95% CI = 0.26-0.68), 4.1-6 OR 0.23 (95% CI = 0.13-0.41) and 2.1-4 OR 0.24 (95% CI = 0.10-0.60). The predictive capacity of the selection process for success in the RACP clinical examination within the minimum time period is high.

Voltage-frequency dependence of the complex dielectric and electric modulus and the determination of the interface-state density distribution from the capacitance-frequency measurements of Al/p-Si/Al and Al/V2O5/p-Si/Al structures

The energy distribution of the interface states (Nss) and relaxation time (τ) are calculated from the capacitance-frequency (C-f) characteristics for Al/p-type Si/Al metal-semiconductor and Al/V2O5/p-type Si/Al metal–interfacial layer–semiconductor diodes with and without anodic surface passivation. The experimental results show that the density of the interface states and the relaxation times increase almost exponentially with the bias from the top of the valence band to the center of the gap for each diode produced. At the same time, using the C-f characteristics, the dielectric properties such as the dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tanδ), real and imaginary portions of the electric modulus (M’ and M″) and ac electrical conductivity (σac) are investigated in this study. The analysis was performed at room temperature, in the frequency range from 1 kHz to 10 MHz, and the voltage range from 0 to 0.24 V. The experimental results show that the ε′, ε″ and tanδ values decrease with increasing frequency while σac, M′ and M″ values increase. This results will show that dielectric parameters are strongly frequency dependent.

(Invited) Luminescent InP Quantum Dots: They're Not Just Like CdSe

Time-resolved photoluminescence (PL) and transient absorption (TA) spectroscopy have been used to elucidate the electron and hole dynamics in high quality InP/ZnSe/ZnS quantum dots (QDs) at room temperature. These dynamics depend critically on the overall In:P ratio. Stoichiometric QDs (those having an In:P ratio close to unity), have PL quantum yields of > 90% and exhibit dynamics that are very simple: both electron and hole relaxation to the bandedge is fast compared to the ~ 40 ps time-correlated photon-counting instrument response and the PL follows a single exponential decay having a time constant of about 26 ns. However, QDs having an In:P ratio greater than about 1.1 maintain PL quantum yields > 90%, but show much more complicated PL kinetics. In the non-stoichiometric QDs, a significant fraction of the PL exhibits a slower risetime. The PL rises over a range of timescales varying from close to the instrument response to approximately 2.0 ns. The time constant of the slowest component increases with increasing InP core size and the fraction of the PL that has the slow risetime increases with excess indium in the particle. The slow risetime is absent in the TA results, indicating that the slow dynamics may be assigned to relaxation in the valence band. The risetime increases with the thickness of the ZnSe shell, varying from < 40 ps for the thinnest (1.1 nm) to about 2.0 ns for the thickest (2.7 nm) ZnSe shells. In addition to the slow risetime, the QDs having excess indium show a significant long-lived (>100 ns) decay component. The magnitude and time constant of this slow decay also increases with ZnSe shell thickness.These results suggest that holes are transiently trapped at sites associated with indium dopants in the ZnSe shell. The trapped hole has negligible overlap with the conduction band electron, making this an optically dark state. The holes are in an equilibrium between the shell traps and the valence band, giving rise to the delayed emission. Several possible assignments of the trapping species can be considered, and a substitutional indium adjacent to a zinc vacancy, In3+/VZn 2-, is the most likely. This assignment is consistent with the observation that trapping occurs only when the QD has excess indium and is supported by experiments showing that the addition of zinc oleate or acetate decreases the extent of trapping, presumably by filling some of the vacancy traps. Further experiments show that addition of alkyl carboxylic acids causes increased trapping, presumably by the formation of zinc oleate, thereby creating additional zinc vacancies. Chloride ion is a common impurity in the synthesis of InP/ZnSe/ZnS QDs and the possibility of In3+/Cl- impurities acting as the hole trap can be also considered. However, additional experimental data on InP/ZnSe/ZnS QDs synthesized in the absence of chloride show the same PL risetimes and delayed emission, eliminating this possibility. Density functional theory calculations further corroborate assignment of the trapping species to In3+/VZn 2-. These calculations show that either a single In2+ ion or an In2+-In3+ dimer is much too easily oxidized to form the reversible traps observed experimentally, while In3+ is far too difficult to oxidize. However, a zinc vacancy adjacent to a substitutional indium is calculated to have its highest occupied orbitals about 1 eV above the top of the valence band of bulk ZnSe, in the appropriate energy range to act as reversible traps for quantum confined holes in the InP valence band.

Determination of the Complex Refractive Index of GaSb1−xBixby Variable‐Angle Spectroscopic Ellipsometry

Variable‐angle spectroscopic ellipsometry is used to determine the room temperature complex refractive index of molecular beam epitaxy grown GaSb1−xBixfilms withx ≤ 4.25% over a spectral range of 0.47–6.2 eV. By correlating to critical points in the extinction coefficientk, the energies of several interband transitions are extracted as functions of Bi content. The observed change in the fundamental bandgap energy (E0, −36.5 meV per %Bi) agrees well with previously published values; however, the samples examined here show a much more rapid increase in the spin‐orbit splitting energy (Δ0, +30.1 meV per Bi) than previous calculations have predicted. As in the related GaAsBi, the energy of transitions involving the top of the valence band are observed to have a much stronger dependence on Bi content than those that do not, suggesting the valence band maximum is most sensitive to Bi alloying. Finally, the effects of surface droplets on both the complex refractive index and the critical point energies are examined.

Structural and electronic properties of GaN: Ab initio study within LDA and LDA+U methods

Structural and electronic properties of the GaN were simulated based on Density Functional Theory implementing Local Density Approximation methods. Hubbard U correction gives us an opportunity to find the correct energy gap for GaN in agreement with known experimental results. Choosing more accurate investigation methods leads to calculating accurate electronic band structure and in the future predicting some physical properties of related material. The bottom of the conduction band and the top of the valence band are formed mainly by p-orbitals of host Ga and N atoms. The present study shows the direct band gap character of GaN with a wurtzite structure

Local bandgap narrowing in the forming state of threshold switching materials

Threshold switching (TS) materials, such as amorphous chalcogenide, have received significant attention for their application in storage class memory and in-memory computing. These materials contribute to efficient data processing and reduced power consumption in data centers. The initial switching process after fabricating a TS device, known as “forming,” has a profound impact on its subsequent TS behavior. However, it remains unclear how TS materials undergo changes in their atomic and electronic structures during the forming process. Consequently, the key factors that govern TS behavior remain obscure, necessitating a deeper understanding of the underlying physics behind TS phenomena. In this Letter, we investigated the forming state of the TS material AlTeN by combining scanning internal photoemission microscopy (SIPM) and ab initio calculations. Thanks to nondestructive evaluation by SIPM measurements, we observed local bandgap narrowing of AlTeN after its forming process. This is an experimental demonstration showing the presence of nuclei of the conductive filament formed in its ON state. Moreover, we conducted an ab initio calculation to reveal the origin of bandgap narrowing. We applied strong electrothermal stresses to the AlTeN model by ab initio molecular dynamics simulation with high electronic and lattice temperatures. By quenching from the electrothermal stress conditions, we reproduced an experimentally observed forming state with a narrowed bandgap. Analysis of the electronic structures of the forming state revealed that the origin of bandgap narrowing is the generation of the valence band top and conduction band bottom stemming from the increased homopolar bonds.

Analysis of energy bands and density of states of CuFe(SxSeyTe1-x-y)2 by calculating

Copper based sulfur compounds have excellent optoelectronic properties which can be regulated through chemical doping, and are the candidate materials for solar cells, photocatalysis and other fields. The crystal models of CuFe(SxSeyTe1-x-y)2 (x = 0.125,0.25,0.25,0.5; y = 0.125,0.25,0.5,0.25) are established, and the energy bands and density of states of CuFe(SxSeyTe1-x-y)2 are calculated using first principles. The analysis results show that as the doping of S atoms increases, the lattice constants a and b gradually increase, while the lattice constants c and cell volume V become smaller and smaller; As the doping of Se atoms increases, the lattice constants a and b gradually increase, while the lattice constant c and cell volume V become smaller and smaller. The valence band top and conduction band bottom of CuFe(SxSeyTe1-x-y)2 (x = 0.125,0.25,0.25,0.5; y = 0.125,0.25,0.5,0.25) are at the same point G. It can be explained that CuFe(SxSeyTe1-x-y)2 is a direct bandgap semiconductor, and the bandgap widths of the selected crystal structures corresponding to the values of x and y are approximately 0.98, 0.97, 0.91 and 0.98 eV, respectively. The diagram for the density of states has peaks on both sides of the Fermi level, and other peaks in the density of states figure also correspond to the band graph. The size and trend of the peaks reflect the strength of electron delocalization.

Study on the Infrared and Raman spectra of Ti3AlB2, Zr3AlB2, Hf3AlB2, and Ta3AlB2 by first-principles calculations

In this paper, the crystal geometry, electronic structure, lattice vibration, Infrared and Raman spectra of ternary layered borides M3AlB2 (M = Ti, Zr, Hf, Ta) are studied by using first principles calculation method based on the density functional theory. The electronic structure of M3AlB2 indicates that they are all electrical conductors, and the d orbitals of Ti, Zr, Hf, and Ta occupy most of the bottom of the conduction band and most of the top of the valence band. Al and B have lower contributions near their Fermi level. The lightweight and stronger chemical bonds of atom B are important factors that correspond to higher levels of peak positions in the Infrared and Raman spectra. However, the vibration frequencies, phonon density of states, and peak positions of Infrared and Raman spectra are significantly lower because of heavier masses and weaker chemical bonds for M and Al atoms. And, there are 6 Infrared active modes A2u and E1u, and 7 Raman active modes, namely A1g, E2g, and E1g corresponding to different vibration frequencies in M3AlB2. Furthermore, the Infrared and Raman spectra of M3AlB2 were obtained respectively, which intuitively provided a reliable Infrared and Raman vibration position and intensity theoretical basis for the experimental study.